Electron lens assembly



Nov. 2, 1948.

c. H. BACHMAN ELEcTRoN LENS ASSEMBLY Filed May 3l, 1946 Ill/lll- 'lll/lill lrwvenJizJcrr` Charles H. Bachmann, bfg m11 TM His Attorneg.

Patented Nov. Z, 1948 UNITED STATES PATENT QFFICE LCTRN LES ASSEMBLY Charles III'. Bachman, Scotia, Y., assigner to General Electric Company, a corporation of New York Application May 31, 1946, Serial No. 673,218

(Cl. Z50-162) that the lens be of such geometry that it 'gives both greatest possible electrical safety and yet is easy to assemble and assures accurate alignment. Furthermore, in order that the aberrations of the lens be a minimum, it is essential that the apertures of the various electrode ele ments be centered on the optical axis and that the electrodes themselves be accurately parallel. Accordingly, it is another object of my invention to provide a new and improved electron'lens system in which centering of the apertures of the various electrodes is obtained with accuracy and facility.

'It is still another object of my invention to provide a new and improved electron lens assembly in which the leakage path through the supporting insulators for the various electrodes is considerably greater than the physical spacing of the electrodes.

It is a further object of my invention toprovide a new and improved electron lens assembly which employs insulators which require accuracy in one dimension only.

One of the features of my invention is the use of a plurality of cylindrical insulators to support and space the electrodes of an electron lens assembly. The individual insulators need be accurate in one dimension only. Yet the assembly provides both axial alignment of the electrodes and centering of all the electrodes along the optical axis of the assembly.

The objects and advantages of my invention may best be understood by reference to the following description taken in connection with the accompanying drawing and the features of novelty which I desire to protect are pointed out with particularity in the appended claims. In the drawing Fig. 1 is a vertical sectional view of an insulator suitably embodying the invention; Fig. 2 is a cross-section of the assembly of Fig. l taken along the line 2 2; Fig. 3 is a vertical sectional view of a modification of the assembly of Fig. 1; and Fig. 4 is a cross-sectional View of the assembly of Fig. 3 taken along the line 4 4.

Referring to Figs. 1 and 2 jointly, I have there shown an electron lens assembly which comprises 2 a metallic outer cylinder l which is provided with an accurately machined inner bore and a plu rality of disk-like electrodes 2 11 which are fitted within the cylinder I. The outer electrodes 2, vd may project inwardly toward the center electrode 3 a distance suilicient to provide desired spacing between the electrodes so that a desired lens effect is obtained. The electrodes 2 4 are provided with respective center apertures 5 1, the central aperture 6 in the intermediate electrode being larger than ythe apertures 5, 1. The electrodes 2 4 may be formed of any suitable conducting material, for example chrome-plated brass, and for the purposes to vbe pointed out later have accurately machined outer surfaces. In this fashion, the electrodes 2, 4 fit smoothly on the inner surface of the cylinder I.

In order that the electrodes 2 4 may be accurately aligned axially and centered with respect '1 to the optical axis of the assembly, I provide three cylindrical insulators 8 I I). While the insulators B Il) may be formed of any suitable material, such as porcelain, steatite, or Mycalex, I have found glass of the type known as Pyrex to be especially suitable in an electron lens assembly. The insulator 8 is ground or prepared on itsV outer surface to fit smoothly within the cylinder l-and is provided with a centrally located raised portion I I which is carefully prepared with an inner surface f2 which is concentric with the outer surface of the insulator 8. The electrode 3 is arranged to fit on the surface i2 so that theaperture 6 is held in accurate axial alignment within the cylinder I. The insulating spacers 9, I0 are carefully prepared with their ends parallel and with accurately measured lengths. Preferably, the length of the insulators 9, I!) plus the thickness of the electrode 3 is greater than the length lof the insulator 8. Such dimensioning and proportion of thickness of insulator S and of the length of insulators S, Il) provides for ease in assembly of the lens and accuracy in alignment,

both axially and with respect to the optical axis of the lens. The dimensions of the diameters of the cylindrical insulators 9 and I0 are not criti- Thus, these insulators are free to move in a direction transverse to the axis of the cylinder I. lj

cal.

4 and which engages a stop, shown as the head of a screw I4 threaded into the wall ot cylinder I. At the opposite end of the assembly the cylindrical sleeve l5 provides means for holding the various elements of the assembly in fixed position. Thus, when pressure in a downward direction is exerted on the sleeve I5, this sleeve engages the outer edge of the electrode 2. Pressure is transmitted through the insulators 9, Ill and the electrodes 3', 4 against the retaining ring I3. In an electrostatic lens' of this type, the electrodes 2, 4 are maintained at the potential of cylinder I, being in conductive contact with that cylinder. Operating potential for the electrode 3 is supplied thereto by a longitudinally extending conductor I6 which is connected to the electrode 3 andA passes through an aperture (not shown) in the outer electrode 4.

In the assembly of Fig. l, the insulators 9, lil, and I I provide a relatively long leakage path b etween the outer electrodes and the center electrodes 3. The outer electrodes have centrally positioned symmetrically Icup-shaped portions which are directed inwardly toward the center` electrode to` effect a configurationof the electric iieldin the vicinity `of, the, apertures Y5 1 to` obtain the lens action desired. Furthermore, the insulator 8 at. the point of greatest potential gradient, its point oif contact with `the center electrode 3, is.- made thicker thanat any` other point to inhibit electrical breakdown by rupture of the insulatingmaterial. The accurately machined outer edges of the electrodes 27, 4 togetherv with the accurately machined |bore of cylinder, IV

assures centeringof the apertures of these electrodes With respect-to the optical axis of the system. Similarly, centering of the electrode 3 ongthe optical axis is provided zby the accurately centeredsurface |2.on the insulator 8.. Asa result, aberration of the composite. lens. structure is held to aminimum. Axial alignment of the respective electrodes is maintained by the accurately dimensioned lengths ofthe. insulators 9. I0..

In the compound cylinder type electrostatic lensassemb-ly of Figs. 3 and 4, I have shown a structure which issuitable for applications in which it is not convenient to use the symmetrically cup-shaped electrodes 2, 4, shown in Eig. l.

Thus, in an electron microscope for example, itv

is, desirable-y to pla-ce an object which isbeing examined asiclose to the lens as possible-and still 'be able to move that object freelyin space relative to thelens for focusing, as well as examining selected areas of the' object. To provide thisA desired freedom, the structure Vof the assembly orf Figs. 3, 4 is made asymmetrical, the outer electrode Il' being made planar and the intermediate electrode I8 and outer electrode I8 ibeing providedwith asymmetrical centrally positioned cup-shaped portions. These electrodesare assembled Within the cylinder I and employ the same` insulators 8--I0 which arek used inthe assembly of.l ig. 1 t0y provide both Yaxial alignment ofrthe respective electrodes and centering ofthe intermediate electrode I8. In thi-s assembly I have shown theretaining ring I3 andscrew 11i aspositioned at the end closer to the apertures of the electrodes in order that an object tobe examined may be` positioned as closeto the-aperture 5 as ispossible. The sleeve lI5 is therefore positioned atthe yopposite end of they assembly.

An important advantage of my improved lens assembly is that the dimensions ofthe insulators employed for supporting and aligning the elec.- trodes needbe accurate in one direction only. Thus the insulator 8 requires accurate dimen-` sioning only of itsouter diameter and of the surface I2 while the insulators 9, I0 are accurately dimen'sioned in length only. By employing a pair of offset lenses, in conjunction with la combination of such insulating cylinders, and by controlling the amount of offset, a long electrical leakage path between electrodes is provided. At the same time, the assembly of insulators provides accurate centering with respect to the optical axis and accurate spacing axially of the respective electrodes. In this fashion, not'l only are the electrical characteristics of the system, so far as breakdown is concerned, improved, but at the same time the Ioptical properties of the lens are enhanced.

While the invention has been described by particular embodiments, it will be understood that numerous modifications may ibe ,made by those skilled in the art Without departing from the invention and I therefore aim in the appended claims t-o cover all such equivalent variations as fall within the true spirit and scope of,v my in- Ventron.`

What I Iclaim as new and desire to secureby. Letters Patent of the United States, is.:

l. An electrostatic electronY lens 'structure comprising a sup-porting cylinder, three circular electrodes arranged 4coaxially within said cylinder and provided with lcentral apertures, theA outer of said electrodes being supported directly. by.

said cylinder and having their apertures aligned thereby, first insulating means of accurately di,-v mensioned thickness for supporting the inter-A mediate ofsaid electrodes within-.said cylinder and for aligning fthe apertures thereof withI the apertures of said outer electrodes, and-additional insulating means of accuratelyV dimensionedlength positioned between saidouter electrodes.

third of said electrodes, rstinsu-lating means4 of accurately dimensioned thickness for. supporting the intermediate of said electrodes within said cylinder and for aligning the apertures thereof with the apertures of said -outer electrodes,y and' additional insulating means of accurately dimensioned length positioned between said outer yelectrodes and said intermediate electrode fory spacing said electrodes axially within said cylin-. der.

3. An electrostatic electron lens structure comprising a Isupporting cylinder, three circularlyV symmetrical electrodes arranged coaxiallywithin said cylinder, the outer of said electrodes being supported directly by said cylinder and having their apertures aligned thereby, means includingy an insulator of accurately dimensioned thickness for centering the intermediate of said-electrodes on the optical axis of said structure andfcr aligning the apertures thereof with theapertures of said outer electrodes, 4and means includinga pair of insulators of accurately dimensioned length positioned between said `outer electrodes andjsaid` intermediate electrode `for aligning said electrodes 'axially of said cylinder.

4. An electron lens comprising a supporting` cylinder, threegcircularly symmetrical electrodesV spaced coaxia-lly within said cylinder, the outer 6 of said electrodes lbeing supported directly by said cylinder and having their 'apertures aligned REFERENCES CITED thereby, two of said electrodes having centrally yThe following references are of record ln the located oiset portions extending toward the third file 0f this Patenti of said electrodes, mean-s including an insulator l of accurately dimensione'd`r thickness for center- UNITED STATES PATENTS ing the intermediate of said electrodes with re- Number Name Date spect to the opti-cal axis 'of said lens, and means y 2,170,663 Painter v Aug. 22, 1939 including 'a pair of insulators of accurately die 2,184,821 Uhlmann Dec. 26, 1939 mensioned length positioned-between said outer l0 vr 2,185,807 GaJbor et al'. Jan. 2, 1940 electrodes and said intermediate electrode for 2,330,930 Snyder. Jr Oct. 5, 1943 aligning said electrodes axially of said cylinder. 2,347,965 Bamberg May 2, 1944 CHARLES H. BACHMAN. l2,400,332 Bachman May 14, 1946 

